Participants consumed a mean of 73.36 g of free sugar, which substantially exceeds the UK dietary recommendation of a maximum intake of 30 g per day in this age group [25]. This finding reinforces existing research which suggests that adolescents consume substantially above the recommended amount of free sugar and highlights the importance of developing effective interventions which reduce free sugar intake in this population group [26,27,28].

Adjusting for energy intake, free sugar intake was highest during snacking occasions, suggesting that foods and drinks consumed in between main meals are likely to be higher in free sugar than main meals. An analysis of the UK National Diet and Nutrition Survey (NDNS) data from 2008 to 2016 found that the highest adolescent consumers of sugar were those with the highest intakes of sweetened drinks, fruit juice, cakes, biscuits, sugar and sweet spreads, and chocolate and sugar confectionery [28], which are foods and drinks that are likely to contribute to intake at snack times. Of the main meals, we found that adolescents consumed more free sugar at breakfast than at lunch and dinner after adjustment for energy intake, indicating that foods and drinks high in free sugar may also be a key feature of breakfast. This is consistent with the findings of another analysis using NDNS data from 2014 to 2016, which reported cereals and cereal products (including breakfast cereals) to be a major contributor to free sugar in adolescent diets, as well as sugars, preserves and confectionery, and soft drinks [29]. Breakfast cereals are a popular choice of breakfast food for children and adolescents, but many contain high levels of free sugar [30, 31] which may help to explain our findings.

Most pupils reported consuming a snack or drink outside of mealtimes, which represents an opportunity for intervention to either reduce snack consumption overall, or promote healthier, lower sugar snack/drink items. Breakfast consumption should also be a target for intervention in this age group, although unlike snack intake, regular breakfast consumption is important for overall health and development in children and adolescents [32]. In our study, only three quarters of participants reported having breakfast. Interventions targeting breakfast need to both encourage regular consumption of breakfast and promote consumption of foods and drinks that are lower in sugar. One strategy to address the latter is food reformulation. In the UK, the Soft Drinks Industry Levy (SDIL) has had some effect in prompting the industry to reformulate their products. An interrupted time series analysis of changes in soft drink purchasing before and after the introduction of the SDIL demonstrated a 8 g reduction in sugar intake per household per week despite an increased volume of soft drink purchased [10]. A similar approach could be applied breakfast cereal manufacturers, whereby they are incentivised to lower the sugar content of their products [31]. The UK government introduced a voluntary sugar reduction programme for the food industry in 2017, but this resulted in only minimal reductions in the sugar content of targeted foods [33]. Another potential intervention to target breakfast consumption in this age group is universal school breakfast provision. If implemented in combination with school food standards that limit the availability of foods and drinks high in sugar, this strategy could promote regular consumption of healthy breakfast foods in this age group. In England, the government are introducing universal free breakfast clubs across all primary schools (children aged 4–11 years) [34] but this does not currently extend to secondary schools.

Free sugar intake outside of school was marginally higher than inside school, which indicates that foods and drinks consumed outside of the school day may be greater contributors to high free sugar intakes. An Australian cross-sectional study conducted with secondary school students found that snacking behaviours were highest in the after-school period [35], which may explain the higher free sugar intake outside of school that we observed. It may, therefore, be important to address snacking and drinking behaviours outside of the school day to effectively reduce intakes of free sugar in adolescents. To date, most interventions aimed at reducing sugar intakes in adolescents have focused on the school setting [36].

SSB intake was higher outside school, but a similar number of confectionery items were consumed inside and outside of school. The current English school food standards [11] prohibit the sales of SSBs and sugar and chocolate confectionery in school, and the limited availability of these items may contribute to reducing consumption of these foods and drinks in school. However, the school food standards do not apply to food and drink items brought into school by pupils. In addition, our evaluation of secondary school compliance with the national standards (undertaken as part of the FUEL study [21]) indicated variable compliance with confectionery-related standards. These factors may help to explain the similar intakes of confectionery inside and outside of school.

To address adolescents’ sugar intake both in and out of school, a systems approach that encompasses all the environments that adolescents interact with is required [37]. Schools are vital partners in this approach, but need to be supported by other agencies and structures to effect change in adolescents’ dietary intake.

Key strengths of the study were the large regional sample, demographically representative of the English population. Dietary intake was measured by 24-h recall, a suitable method for this age group [22] and in nearly half of the participants, we measured dietary intake on two separate days. Limitations included the accuracy of self-reported dietary intake. Underreporting is a known issue in all self-reported dietary assessment, particularly for some nutrients e.g. energy intake [38]. As a result, the adjustment of our models for energy intake may not have fully accounted for its influence upon sugar free intakes. However, underreporting with Intake24 is similar to that using interviewer-led 24-h recall [18]. The use of pre-defined mealtime labels (breakfast, lunch, evening meal, snack/drink) may not fully reflect adolescent eating patterns, however, this was arguably the most practical approach available. The methods we used for defining dietary intake inside and outside school may have led to some misclassification, as these were based in part on time of consumption during the day. However, the impact of this upon the results is likely to be minimal.

Given the findings of this study, it is recommended that future interventions and policies to reduce free sugar intake among adolescents focus on snacking occasions (including drinks outside of mealtimes) and breakfast. There is also a need to focus future intervention efforts outside of the school setting, particularly given that most interventions targeting this age group to date have focused on schools and school food. A further understanding of the relative influence of home versus other out-of-school contexts upon dietary behaviours would help in developing more effective strategies to reduce free sugar intakes in this age group.